Keratins cuticle

The egg shell is 94% calcium carbonate [471-34-17, CaCO, 1% calcium phosphate [7758-23-8] and a small amount of magnesium carbonate [546-93-0]. A water-insoluble keratin-type protein is found within the shell and in the outer cuticle coating. The pores of the shell allow carbon dioxide and water to escape during storage. The shell is separated from the egg contents by two protein membranes. The air cell formed by separation of these membranes increases in size because of water loss. The air cell originally forms because of the contraction of the Hquid within the egg shell when the temperature changes from the body temperature of the hen at 41.6°C to a storage temperature of the egg at 7.2°C. 

The average hair is composed of about 90% keratin, a fibrous protein, and about 10% water. Each hair consists of two or three layers. The outer layer is a sheath called the cuticle, which is thin and colorless. It is made up of overlapping scales in layers. Inside the cuticle are the long spindle-shaped cells of the cortex, which makes up the bulk of the hair. These cells also contain whatever pigments are present in the hair. The innermost layer, which is only present in large thick hair, is the medulla or pith. 

Appendageal structures commonly found within the skin are the hairs, hair follicles, associated sebaceous glands, apocrine and eccrine sweat glands, and arrector pili muscles. Hairs are formed by epidermal invaginations. These keratinized structures traverse the dermis and may extend into the hypodermis. The free part of the hair above the surface of the skin is the hair shaft, and the part deep within the dermis is the hair root, which forms an expanded knob-like structure called the hair bulb. This is composed of a matrix of epithelial cells in different stages of differentiation. Hair is composed of three concentric epithelial cell layers the outermost thin cuticle, a densely packed keratinized cortex, and a central medulla of cuboidal cells. The hair follicle consists of four major components (1) internal root sheath (internal root sheath cuticle, granular layer, pale epithelial layer) (2) external root sheath (several layers similar to the epidermis) (3) dermal papilla (connective tissue) and (4) hair matrix (comparable to the stratum basale of the epidermis). 

Hair is composed of approximately 65 to 95% protein, 1 to 9% lipid, and small quantities of trace elements, polysaccharides, and water. - - The majority of hair protein is often referred to as keratin, which is a general term used to describe aggregates of protein with low or high sulfur content. These proteins are synthesized in the keratogenous zone of the hair follicle as matrix cells move upward from the hair bulb to form layers of the hair shaft. The cuticle, cortex, and medulla are comprised largely of keratin, although it is structurally different in each layer. Keratin in the exocuticle contains a high concentration of cysteine, which forms disulfide bonds which link the A-layer to the exocuticle, and this makes the cuticle... 

Fibrous protein structure investigations applying X-ray diffraction and electron microscopy were reviewed by Blakely (31). Keratin fibers are made of three main structural components the cuticle, the cortex, and the medulla. The medulla is only present in coarse fibers. The cortex forms the bulk of the fiber. Various morphological models have been proposed to explain the mechanical properties of keratin fibers. It is generally agreed that the cortex consists of fibrils in which protein molecules exist in helical and nonhelical regions. 

The fiber surface is bounded by a thin membrane 100 A thick called the epicuticle. The cuticle is a scaly, tubular layer and consists of flattened cells which overlap to give a rachet-like profile to the fiber. Each scale cell contains two distinct layers, a keratinous outer layer termed the exocuticle and a nonkeratinous inner layer that appears to be derived from cytoplasmic debris and is termed the endocuticle. There is some evidence that the exocuticle itself is complex with an outer cystine-rich layer termed exocuticle a. In coarse fibers the cuticle may be many scale-cells thick and where the cells overlap they are separated by an intercellular layer formed during biosynthesis by the deposition of nonkeratinous protein between the cell membranes. This layer is sometimes referred to as intercellular cement. 

The cortex forms the bulk of fine animal hairs and is derived from highly differentiated spindle-shaped cells that are densely packed with keratinous proteins. The long axes of the cortical cells are oriented parallel to the fiber length, and elongated cavities near the center of the cells are similarly oriented. These cavities are derived from the nuclei of the developing cells and contain debris usually referred to as nuclear remnants. Between cortical cells there is a layer 250-300 A in thickness which is similar to that found between cuticle cells this also is referred to sometimes as intercellular cement. Many nonkeratinous inclusions are found within the cortical cells and these are believed to be cytoplasmic debris. 

True hair is found only in mammals, and there is no such thing as a completely hairless mammal. Hair itself is dead, but is produced in hair follicles by specialized keratinocytes at the base of the hair. The outermost layer of hair is a cuticle, and most hairs have a cortex in which the dead keratinized cells are very densely packed, and an iimer medulla in which they are not as densely packed. The pigmentation in hair, like that of skin, comes from melanocytes. Hair exposure to some chemicals may produce hair discoloration, for example, green hair from copper in water or cosmetics, or blue hair in cobalt workers. 

Lynch MH, O Guin M, Hardy C, et al. Acidic and basic hair/ nail (hard) keratins their colocalization in upper cortical and cuticle cells of the human hair follicle and their relationship to soft keratins. / Cell Biol. 1986 103 2593-2606. 

Wools and other similar mammalian hairs are largely composed of keratin proteins. However, unlike the other natural proteinaceous fibre, silk, wool is cellular in nature the fibres consist of relatively hard, flattened, overlapping cuticle cells, which surround the central cortical cells in some fibres, these may in turn surround a hollow medulla (Figure 23). 

A plausible explanation for these results is that inside the keratin structure water is molecularly dispersed and forms monomolecular layers around the various protein structural units, i.e., the micro-or protofibrils of the keratin. The low values obtained for Vw can be explained by assuming that when water molecules penetrate the hair structure, they fill, at least in part, pre-existing voids. The results also suggest that the cortex of the hair structure is more porous than the cuticle, since removal of the cuticle descaling of the hair) reduces the value of... 

In addition to sclerotinization there is some older evidence suggesting the existence of other mechanisms of protein stabilization. Keratin or keratin-like proteins characterized by high sulfur (presumably cysteine) content have been identified by histochemi-cal methods in several trematode species and may be characteristic of paramphistomes (8,13). Crosslinking by oxidative coupling of tyrosine-residues to form dityrosine may also occur in trematode eggshells (14). Dityrosine crosslinks are well known in nematode cuticles (15) but detailed analyses of tyrosine derived crosslinks in eggshells has not been performed. 

A portion of the undermembrane of Figure 1-21 is also epicuticle. The cystine-rich proteins of the cuticle belong to the group of proteins called keratin-associated proteins. Although structurally different, keratin-associated proteins are also found in the matrix of the cortex. See the section that discusses keratin-associated proteins in Chapter 2. For more details of the intercellular structures, see Figure 1-23. Thus, the cuticle of human hair is a laminar structure similar to the cuticle of wool liber, and the different layers of the cuticle have been described for merino wool [64] and for human hair [58, 65, 66, 67], Figures 1-23 and 1-24 illustrate the... 

The data for cuticle analysis are based on the work of Bradbury et al. [16] who analyzed cuticle and whole fiber from several keratin sources, including human hair, merino wool, mohair, and alpaca. These scientists concluded that there is very nearly the same difference between the amino acid composition of the cuticle and each of these fibers from which it was derived. They listed the average percentage differences used in these calculations. More recent analyses of cuticle and whole fiber of human hair [68, 69] are in general agreement with these data [18]. 

Evans et al. [25] have confirmed these conclusions of Wickett. In addition, the observation that Japanese hair is easy to perm and that fine Caucasian hair, less than 75 pm in diameter, is more difficult to perm was also confirmed. However, these scientists were unable to identify any common characteristics such as fiber diameter or cystine content that would account for this behavior. The fact that fine hair is more difficult to perm than thick hair may be due to the larger ratio of cuticle to cortex in fine hair and the fact that cortex plays a stronger role in waving than cuticle. This explanation is consistent with the experiments by Wortmann and Kure [2], demonstrating that the cuticle does inhibit the reduction reaction. In addition to pH, other important variables that influence the rate of reduction of keratin fibers by mercaptans are temperature, hair swelling, prior history of the hair, and structure of the mercaptan. 

The hair shaft is composed of three layers an outermost cuticle, a cortex of densely packed keratinized cells, and an innermost medulla of loose cuboidal or flattened cells. The cuticle is formed by a single layer of flat keratinized cells in which the free edges, which overlap like shingles on a roof, are directed toward the distal end of the shaft. The cortex consists of a layer of dense, compact, keratinized cells with their long axes parallel to the hair shaft. The medulla forms the center of the hair and is loosely filled with cuboidal or flattened cells. In the root, the medulla is solid, whereas in the shaft it contains air-filled spaces. The pattern of the surface of the cuticular cells, together with the cellular arrangement of the medulla, is characteristic for each species. 

Medullary cells are resistant to attack by normal reagents used for solubilizing keratins [163]. Methods used for the isolation of medulla are based on its inherently inert nature. Thus, medullary isolates result as a residue after preferential dissolution of cuticle and cortical cells [160]. 

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